A fluidized bed is a suspension of solid particles in a stream of gas or liquid of sufficient velocity to support the particle by flow forces against the downward force of gravity. Fluidized beds are critical components of important petrochemical processing units such as the catalytic cracking ("cat-cracking") of petroleum on catalytic particles to produce lighter and more valuable products as well as thermal cracking of heavy feeds on coke particles ("fluid bed cokers" or "flexi-cokers") to again produce lighter and more valuable feeds. In cat-cracking the regenerator where coke is burned off the catalyst to produce "fresh catalyst" contains a fluidized bed. The particles in the fluidized bed within the regenerator are approximately 60 micron diameter pellets of a zeolite. In the case of fluid bed or flexi-coking, fluidized beds can be found in the heater, reactor and in the case of flexi-coking, the gasifier. The particles in this case are approximately 100 to 150 micron particles of coke.
Other fluidized beds containing small solids suspended in a gas include advanced coal combustion units where small particles of coal are suspended and burned to produce heat with minimum pollution and maximum efficiency. Yet another example is found in separation processes in the chemical industry where a fine suspension of particles is suspended in a flowing liquid. In general, fluidized beds are used in many large scale processes where it is desired to maximize the interaction between the surface of a particle and a surrounding gas or liquid.
Fluidized beds can contain volume mass densities for the case of fluid bed cokers and regenerators of the order of 40 pounds per cubic foot and particle velocities of several feet per second. Fluidized beds of the order of 10 to 50 feet in diameter are found in coking and cat cracking. With bed heights of the order of 10 to 60 feet the contained fluids range from less than a hundred to more than a thousand tons of particles.
It is difficult to directly measure the bed height in these units because they are operated at high temperatures, usually have a refractory lining and the reactants tend to foul probes placed in the bed. As a result, the bed height is usually inferred from pressure drop measurements across the bed or from gamma ray absorption through the bed. The former is unreliable as discussed in detail in the following paragraph and the latter is a complex refinery safety problem.
The bed level (the height of the transition between the "dense" phase of the fluidized bed and the "dilute" phase) is an important variable in the overall process yield. This quantity is usually inferred from pressure drop measurements. The measurement is difficult for two reasons: the first is the above mentioned fact that the measurement itself is complicated by the possibility of fouling of the measurement tube and pressure artifacts by the bridge arrangement required to eliminate the pressure drop of the inert gas flow used to maintain the opening in the probe. A more fundamental reason is found in the inadequacy of the model used to infer bed level from pressure drop. The general principle is a relationship between the total weight of the bed and the pressure drop. For single phase fluid, where the fluid mass density is well known the connection is immediate and the fluid level is given by the pressure drop divided by the density of the fluid and the acceleration due to gravity. For the case of fluidized beds, the interpretation is less direct since the bed mass density may not be constant but may depend on flow conditions, such as the distribution of fluidization gas, or instabilities of the fluidized bed such as gas bubbling. There are many examples when bed heights have been so poorly monitored that conversion was adversely effected.
Accurate measurements of bed level in addition to monitoring agreement with design conditions, are often used as a diagnostic tool to determine fluidization failures such as "bed slump" where one side of the fluidized bed is not fluidized, or regions of excessive turbulence when fluidization gas is not distributed uniformly. In both cases the efficiency of the petrochemical process is greatly effected by the poor flow state of the contained two phase fluid. In the second case, the high particle velocity can lead to the loss of excessive fines into the atmosphere as well, or excessive temperature gradients within the bed. Finally if the bed height extends into the region of the vessel containing the cyclones, it can interfere with the proper functioning of the cyclones and produce excessive carry over of particles into the gas stream exiting the unit.
While pressure, temperature and net volume or mass flow are the normal way of monitoring the state of fluidization within a fluidized bed or while a unit is operating, there are a variety of techniques that can be brought to bear on functioning fluidized beds. One example is the use of gamma rays or neutrons to determine the mass density of particles within the vessel. This technique can only be used if the walls and/or diameter of the vessel are less than a critical value since the technique is based on deriving the density from absorption. Too large a vessel diameter, or too thick a wall drops the detected signal below the level of background noise and the mass density cannot be determined. In addition the presence of intense radioactive sources and the necessity to construct elaborate structure to support the detectors of the radiation reduce the use of this technology to elaborate field tests or where major uncertainties arise over the operation of the fluidized beds. The gamma or neutron technique is expensive, has to be scheduled in advance and usually beyond the capability of normal refinery personnel.
Non-intrusive probes that can be used to monitor the flow state of experimental fluidized beds would also be of great value in complementing visual, radiographic and radioactive tracer studies of flow in order to improve or modify existing designs, or for pilot plant studies. A current review of a wide variety of electrical, optical, thermal and mechanical technology for studying the hydrodynamics of experimental gas-solid fluidized beds is contained in a recent review by N. P. Cheremisonoff (Ind. Eng. Chem. Process Dev. 25, 329-351 (1986)). The review presents techniques that are "best suited for laboratory scale systems, [although] adaption to industrial pilot facilities and/or commercial units is possible in some cases". However, examination of the presented techniques suggest they suffer from the usual disadvantages of being intrusive, easily contaminated by the process or as in the case of so many of the radioactive techniques severely restricted by environmental or safety considerations.
In the July 1985 issue of the Journal of the American Society of Lubrication Engineers (Lubrication Engineering), J. W. Spencer and D. M. Stevens (of Babcock & Wilcox, a McDermott company of Lynchburg, Va.) describe a technique for "detecting and characterizing particulate matter in fluid flow systems" by using "acoustic emission technology". In this technology the impact of particles generates high frequency surface vibrational waves which are detected as "pulses" by resonant piezo electric transducers. As described in the article, only sensors in contact with probes inserted into the flowing stream correlated with bulk quantity or size of particles in the stream. Sensors mounted non-intrusively on the walls of the pipe "did not correlate well with probe-mounted transducers. Again this technique is intrusive since it requires penetration of the walls of the vessel (see also U.S. Pat. Nos. 3,816,773 and 4,095,474 which describe similarly intrusive techniques).
Thorough review of the prior art suggests that there are no known technologies for reliably and safely measuring or inferring the flow state of two phase flow within a fluidized bed that meet the following criteria:
(1) Non-intrusive and hence requiring neither penetration of the wall or the constructing of external frame works to support radioactive sources and detectors and hence permitting trouble shooting of commercial units;
(2) Non-radioactive and/or suitable for "on-line" monitoring of fluidized beds or transfer lines on working commercial units;
(3) Capable of applying in a "non-intrusive manner" to the refractory lined vessels and transfer lines containing solid particles in the presence of gases such as air, steam and/or volatile hydrocarbons with wall temperatures as high as 250.degree. to 500.degree. C.